The present invention relates to rolls, and in particular to deflection-compensated rolls, of the type which are adapted to be used in the calender or press section of a paper machine.
Thus, the present invention relates to a roll compensated with respect to its deflection and on which a load is imposed at the ends of the roll. The roll has a non-rotating roll axle at the ends of which the load is applied, while a shell of the roll rotates about the axle thereof.
The deflection-compensated roll of the invention, which may have the load imposed at the ends thereof, is intended to be used in the pressure treatment of web-like sheet material while the latter passes through a nip defined between the roll of the invention and a mating roll. Thus, the roll of the invention is particularly suitable for use as a press roll or calender roll in a paper machine.
It is in general well known that rolls of the type referred to above, particularly when loaded by imposing a load at their ends, suffer deflection so that the linear pressure at the press nip is greater at the region of the outer ends of the nip as compared with the central region of the nip. Of course, attempts have already been made to avoid these drawbacks, for example by providing such rolls with a convex exterior surface. With conventional convex rolls, however, it is possible to obtain, for example, a uniform linear pressure only with a given load. In order to avoid this latter drawback it has already been proposed to provide controllable convex rolls an example of which are the known Kusters rolls (U.S. Pat. No. 2,908,964), wherein a pressure fluid chamber is situated between the stationary roll axle and the outer rotating roll shell at a given sector thereof. By controlling the pressure in such a pressure fluid chamber it is possible to compensate the deflection of the roll shell. However, such Kusters rolls suffer from the drawback of having sealing difficulties accompanied by unavoidable leakage of pressure fluid. A further drawback of these Kusters rolls resides in the fact that they respond relatively slowly to changes of pressure, an interval of approximately 30 seconds being required for an increase in pressure while an interval of approximately 10 seconds is required for a decrease in pressure to be effected. The consequence of these latter drawbacks is that when a change in pressure is brought about a considerable quantity of paper is wasted when such a roll is used in a paper machine.
Furthermore, there are known deflection-compensated rolls such as the so-called CC rolls wherein pressure shoes frictionally rub against the inner surface of the roll shell, a load being imposed on these shoes by way of a pressurized fluid. However, in the same way as with the Kusters rolls, such a construction also presents the drawback of having sealing difficulties, and in addition it is relatively difficult to provide for controls capable of achieving asymmetrical effects axially of the roll.
Press rolls which make use of electromagnetic forces are also known. In this connection reference may be made to U.S. Pat. No. 3,456,582. However, the roll disclosed in this latter patent is not loaded at its ends and is only of limited use in that it requires the press roll to have a magnetic roll cooperating therewith, with the press roll carrying magnetic structure for controlling the pressure with which the mating magnetic roll cooperates with the press roll.